1. Field of the Invention
This invention relates to the art of aluminum nitride and more particularly, to a method for producing aluminum nitride fine powder which is high in purity and has good sinterability.
2. Description of the Prior Art
Aluminum nitride is now expected to have wide utility in various fields including not only the field of high temperature heat-resistant materials based on good mechanical characteristics and chemical durability, but also the field of heat-radiating materials in the semiconductive art because of its high heat conductivity, good electric insulting property and low dielectric constant. In most cases, aluminum nitride has been employed as a sintered product although it may be utilized in the form of a thin film.
It is known that the sintering properties and characteristics of an aluminum nitride sintered product is greatly influenced by the characteristics of a starting aluminum nitride powder and the type and manner of sintering agent dispersed in the powder. More particularly, the aluminum nitride powder should be highly pure as well as fine and uniform in size. A suitable sintering agent should be uniformly dispersed in the aluminum nitride powder.
In general, an aluminum powder is produced by a direct nitriding method of metallic aluminum or a reducing and nitriding method of alumina. With the direct nitriding method, it is very difficult to obtain an aluminum nitride powder which is fine and uniform in size and has high purity. In addition, it is also difficult to uniformly disperse a sintering agent in the aluminum nitride powder. If, in the reducing and nitriding method, starting alumina used is fine, uniform and highly pure, it is more likely to obtain an aluminum nitride powder of a slightly better quality than in the case of the direct nitriding method. Nevertheless, such a powder is far from a desired powder.
Several improved methods of obtaining a highly pure and fine aluminum nitride powder with a uniform size have been proposed, for example, in Japanese Laid-open Patent Application No. 61-6105 and Japanese Patent Publication No. 61-2685. Such methods include a method which comprises water to a dispersion of aluminum alkoxide and carbon, causing the alkoxide to be hydrolyzed thereby containing a mixture of aluminum hydroxide and carbon, and a method in which an alkali is added to an aqueous solution containing a water-soluble aluminum salt and carbon to obtain a mixture of aluminum hydroxide and carbon by neutralizing precipitation.
The mixture obtained by these methods are more uniform than the mixture of alumina and carbon. When these mixtures are sintered in a non-oxidizing atmosphere containing nitrogen, there can be obtained an aluminum nitride powder which is more uniform and finer than the powder from the mixture of alumina and carbon.
However, these improved methods are also disadvantageous in that when water or an alkali is added for the hydrolysis or precipitation by neutralization, a precipitate is locally formed and immediately coagulates in situ. This results in formation of a number of aluminum hydroxide aggregates which are free of any carbon fine powder and have a size of not less than 1 micron. Accordingly, the resultant mixture of aluminum hydroxide and carbon is not satisfactory with respect to the uniformity. This leads to a tendency toward the irregularity in size of the aluminum nitride obtained by sintering the non-uniform mixture in a non-oxidizing atmosphere containing nitrogen. After sintering, the alumina is liable to remain non-nitrided with a high possibility of forming coarse grains of aluminum nitride having a size of from 1 to 5 microns.